Scanning tunneling microscopy study of oriented poly(tetrafluoroethylene) substrates

Scanning tunneling microscopy study of oriented poly(tetrafluoroethylene) substrates

Synthetic Metals, 55-57 (1993) 329-334 329 SCANNING TUNNELING MICROSCOPY STUDY OF ORIENTED POLY(TETRAFLUOROETHYLENE~ SUBSTRATE$ ..1 2 P. BODO , C...

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Synthetic Metals, 55-57 (1993) 329-334

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SCANNING TUNNELING MICROSCOPY STUDY OF ORIENTED POLY(TETRAFLUOROETHYLENE~ SUBSTRATE$

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P. BODO , Ch. ZIEGLER, J. R. RASMUSSON I, W. R. SALANECK 1and D. T. CLARK3 1Dept. of Physics, IFM, Link6ping University, S-581 83 LinkOping, Sweden 2Dept. of Phys. and Theor. Chemistry, Eberhard-Karls-University, 7400 Tiibingen, Germany 3ICI, Wilton Materials Research Center, Cleveland TS6 8JE, England

ABSTRACT Highly oriented molecular structures of organic molecules are important in many applications of thin films and interfaces. Recently, it has been shown that poly(tetrafluoroethylene), or PTFE, can be deposited mechanically to form highly oriented thin films on glass substrates. Such oriented PTFE films can in turn be used as substrates for growth of ordered films of organic molecules and polymers, e. g. conjugated polymers. Despite the fact PTFE is electrically insulating, we have been able to use scanning tunneling microscopy (STM) to examine details of oriented ultrathin PTFE films deposited on various substrates, in particularly Pt and highly ordered pyrolytic graphite (HOPG). Ordered structures originating from aligned chains were observed, and reproducible images were obtained at the submicron level. Nanometric molecular details are more difficult to resolve, but indicate interesting features. In particular, we have observed regions of parallel zig-zag shaped molecules, which are separated by approximately 6/~. INTRODUCTION Specific properties of polymers, e.g. mechanical stiffness, strength, and electrical conductivity are often orders of magnitude better in highly oriented species than in isotropic species [1,2]. It is therefore of importance to develop and study materials which may serve as substrates for controlled growth of highly ordered thin films of polymers and organic molecules. Based on early observations in the area of polymer friction and wear [3], Wittmann and Smith [4] have demonstrated a new technique to deposit highly oriented films of poly(tetrafluoroethylene), or VI'FE, which they recently reported can be used as substrates for growth of ordered films of a large number of molecular materials [5], including many that normally do not show a strong tendency to order. In this technique, a bar of PTFE is moved over a fiat substrate surface, e. g., glass, at controlled speed, temperature and contact pressure. Using electron diffraction techniques, it can be seen that the PTFE polymer chains deposited on the substrate are preferentially oriented along the sliding direction, resulting in a ultrathin highly ordered layer of PTFE macromolecules. We have examined the possibility of using scanning tunneling microscopy, STM [6], to image molecular details and electronic properties of mechanically deposited PTFE films. In STM, a Elsevier Sequoia

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sharp and electrically conductive tip is brought near an electrically conductive sample such that an applied bias voltage of a few tenths of millivolts between the tip and the sample results in a tunneling current. Obviously, the use of STM to image PTFE, which is an insulator, requires that the deposited f'llms are thin enough to allow the tunneling current to reach the conducting substrate. Specific problems related to STM studies of organic materials have been discussed and reviewed previously [7]. EXPERIMENTAL The PTFE films were mechanically deposited using a technique similar to that reported by Pooley and Tabor [3] and Wittmann and Smith [5]. Prior to deposition, a solid bar of PTFE and a substrate were heated to a temperature in the range of 100-300"C. Then the PTFE films were deposited by sliding the edge of the PTFE bar over the substrate surface. In this primarily study, we were not able to exactly measure the sliding speed and the pressure of the bar against the substrate surface, but they were estimated to be in the same order of magnitude as those used by Wittmann and Smith [5]. The substrates used for STM studies were clean surfaces of HOPG, Pt, Au and TiN. The HOPG subslxate surface was cleaned by peeling off the top layers by a Scotch tape, the Pt foil was annealed in a flame resulting in a clean surface, and the Au substrates were freshly evaporated thin f i l l s on mica or on optically flat Si wafers. To obtain clean PTFE fdms, the PTFE bar was several times slided over a clean dummy surface of glass before sliding over the actual substrate. The PTFE f i l l s were examined by X-ray photoelectron spectroscopy (XPS), which showed that the mechanically deposited f i l l s were clean except for some hydrocarbon contamination originating from the exposure of substrates to the ambient atmosphere during the deposition process. However, the hydrocarbons could be removed by heating the sample in vacuunl.

After deposition of PTFE, the samples were examined by STM, which in this study was operated in the ambient atmosphere. The STM used is of our own design and construction and has been described elsewhere [7]. All topographic images shown are such that the grey level corresponds to the measured level of the tip while scanned over the sample surface. No image processing or filtering are used, i. e., all images are presented as recorded. Calibration of the lateral scanning area of the STM is derived from the typical hexagonal pattern of HOPG (Fig. la). RESULTS STM images were initially recorded of PTFE f i l l s deposited on HOPG, TIN(111) epitaxially grown on MgO, Au evaporated on SiOx, Au (111) epitaxially grown on mica, and Pt foils. At relatively large scan areas, typically 2-4 lam2, the presence of PTFE could be observed on all these substrates with some different reproducability. For example, on TiN the images often appeared very noisy and on Au the high contamination rate disturbed the imaging conditions. Small isolated islands of PTFE, oriented in the sliding direction, were however seen on the Au and TiN samples. Hence, the experiments were focused upon samples of PTFE on HOPG and Pt, which showed the most interesting results. The most uniform and well aligned PTFE films observed in our large scan images were obtained on the Pt foils.

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Fig. 1. STM topographic images of showing a) the typical hexagonal pattern of the bare HOPG surface and b) PTFE coated HOPG indicating highly ordered polymer chains separated about 6/~. Black-to-white corresponds to a topographic diffcrcnce of about 16 A. The tunneling current and the bias voltage were in a) 0.3 nA and 60 mV and in b) I nA and 50 mV. (Some distortions due to tip instabilities and relaxation drift effects in the piczo scanner are present.) FFFE on H O l ~ HOPG arc useful substratcs for STM studies, because the atomic corrugation of the (0001) cleavage plane is easily r~solved, showing the hexagonal pattern in Fig. la. This pattern can be used as direct reference on the molecular scale. Due to a weak interaction with organic molecules,

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Fig. 2. a) STM topographic image of a clean Pt surface used as substrate for mechanical deposition of PTFE showing typical biaxial grains obtained in a rolled metal foil. b) Pt surface coated with PTFE. In the sliding direction, bundles of oriented polymer oriented chains indicate a highly ordered polymer thin film. Black-to-white corresponds to a topographic change of about a) 500/l~ and b) 1250 ~. The tunneling current and the bias voltages were in a) 1 nA and 50 mV and in b) 1 nA and 100 mV. (Relaxation drift effects in the piezo scanner are present.)

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Fig. 3. STM topographic image of Pt surface coated with mechanically deposited PTFE. its surface is just very slowly contaminated in the ambient atmosphere. A disadvantage is the poor mechanical strength in the bonds perpendicular to the surface, which makes it difficult to mechanically deposit PTFE without damaging the surface structure. Still, XPS measurements revealed the presence of PTFE on the HOPG surface after mechanical deposition, but in many of our experiments it was hard to obtain reproducable STM images showing ordered PTFE films. However, in some cases imaging of ordered regions were successful, and Fig. lb shows features of an ordered structure resembling individual parallel PTFE polymer chains laying flat on the surface and partly covering the hexagonal pattern of the HOPG substrate. The chain features have an orientation corresponding to the sliding direction, are uniformly distributed, and separated by a distance estimated to 6/~. Hence, the features can be assumed to correspond to P'ITE chains rather than artifacts produced in the graphite as a consequence of the deposition procedure. Using scanning force microscopy (SFM) to study mechanically deposited PTFE on glass slides, similar images of parallel PTFE chains have been obtained by Dietz et al. [8], who also reported a distance of about 6/~ inbetween individual chains. PTFE on ~

The surface structure of the Pt foil, used as substrate, shows in Fig. 2a the grain structure typical for rolled metal plates or foils, i.e., small biaxial grains with the long axis parallel to the roiling

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direction. Fig. 2b shows an image after deposition of a PTFE film completely covering the Pt substrate. The film consists of bundles of several PTFE polymer chains running parallel over the whole image area in the sliding direction and perpendicular to the rolling direction indicated in Fig. 2a. Within the bundles, a weak fine structure originating from the underlaying Pt grains can also be seen running perpendicular to the polymer bundles in Fig. 2b. Fig. 3 shows an image of PTFE on Pt, where features of molecular size can be observed. The structure indicates individual parallel PTFE polymer chains separated about 6 /~, which is the same distance as that we observe on HOPG. In the mid and upper part of the image the chains exhibit a zig-zag structure with a repeat length of about 5.5 .~. In the middle of the image the bias voltage was changed from 50 mV to 100 mV, resulting in a slightly improved image quality. The bias dependence is certainly of considerable importance for the STM imaging, especially of insulating substrates such as polymers. A zig-zag pattern, similar to that in Fig. 3, has also been observed by SFM by Dietz et al. [8], and they report the corresponding repeat distance to be about 6/~. The zig-zag pattern is assumed to originate from the helical strucure of PTFE molecules, which has been modelled from electron diffraction studies by Bunn and Howells [9]. According to their model the zig-zag of fluorine atoms is about 8 /~, which is somewhat larger than our measured value of 5.5/~. CONCLUSIONS Using STM, ordered structures in thin films of mechanically deposited PTFE on Pt and HOPG have been observed. As a consequence of the deposition technique, bundles of polymer chains are aligned over large areas. On the molecular scale, we have also observed local ordering of individual PTFE molecules. Also, a helical structure of the molecules is indicated by a fluorine zig-zag structure in STM images resolving individual PTFE molecules. ACKNOWLEDGEMENTS The authors acknowledge the financial support from the Swedish Research Council for Engineering Sciences (TFR), the Swedish National Board for Industrial and Technical Development (NuTek), the Swedish Natural Sciences Research Council (NFR), and the CEC Science program (POLYSURF, grant number 0661). REFERENCES 1 S.Y. Oh, K. Akagi, and H. Shirakawa, Synth. Met.32 (1989) 245. 2 P. Smith and P.J. Lemstra, J. Mater. Sci. 15 (1980) 505. 3 C.M. Pooley and D. Tabor, Proe. R. Soc. Lond. A329 (1972) 251. 4 J.C. Wittmann and P. Smith, US Patent Applic. No. 361 (1989) 129. 5 J.C. Wittmann and P. Smith, Nature. 352 (1991) 414. 6 G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, Appl. Phys. Lett. 40, 178 (1982); Phys. Rev. Lett. 49, 57 (1982). 7 P. Bod6, J.R. Rasmusson, and W.R. Salaneck, in K.L. Mittal (ed.), Metallized Plastics 3: Fundamental and Applied Aspects, Plenum Press, New York, in press. 8 P. Dietz, P.K. Hansma, K.J. Ihn, F. Motamedi, and P. Smith, to be published. 9 C.W. Bunn and E.R. Howells, Nature. 18 (1954) 549.